//1,JavaScript把null,undefined,NaN,0,''(空字符串)视为false,其他值一概视为true
var a = null;
if (!a) {
	console.log("null is false");
}

a = undefined;
if(!a){
	console.log("undefined is false");
}

a = NaN;
if(!a){
	console.log("NaN is false");
}

a = 0;
if(!a){
	console.log("0 is false");
}

a = '';
if(!a){
	console.log("空字符串 is false");
}

a= ' ';
if(a){
	console.log("空格 not is false");
}

a= -0;
if(!a){
	console.log("-0 is false");//-0就是0
}
